Off the top of my head, you need a
servlet container that supports Servlet API 2.3 (so even
Tomcat 4 would be fine), a database like MySQL or PostgreSQL, and Java 5. I wouldn't recommend less memory than 256 MB, although 128MB may be fine (here at JavaRanch we use way more than that, but then, our installation is heavily used).
These issues have probably been discussed at length in the forums over at jforum.net.